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It looks good on paper...

First I must say that getting a few Manual J's and D's gives peace of mind.

I've been going over the ducting, comparing the design(s) of each. I see that the ducting has a column for TEL. Also each branch is listed as a separate measurement. It's great that this is part of the equation and I can grasp the concept as for why.

So my question is, Due to limited space in the attic am I able to streamline or cut down the length(s) of the ducting as designed? I have no problem with resubmitting a fairly detailed print or drawing of the workable routing and lengths that would result.

I understand that rules of thumb are bad and the Manual D was drawn with the TEL in mind. I even tried to self educate by crunching the room requirements as compared to the branch line vs the trunk line vs the cfm vs fpm and so on.

I am totally pleased with the engineered results but unfortunately it will be a PITA to make this work in my situation. It looks good on paper, but...

The operative word in the opening post is, length. Duct diameter is not going to be reduced. Defeats the purpose. I need that diameter to get the air into the room (s).

Can I cut down on the length(s). Will that defeat the purpose.

I think some of you read too fast
Go easy on me, I'm new

We're going easy on you because I think many of us suspect English is not your first language, not because you're new. Which is fine, of course. But if it is your first language - you type to fast - we don't read too fast. No-one in here could figure out what you were asking - and I said the cross section was more important than the length in the first reply!

I am under the assumption that the length(s) of the ducting is part of, not just the reason, of the Man D to help get the airflow down to the rooms requirement.

As dash mentioned, a shorter run will get more air. So this means a longer run will slow it down? If so, will having to take a branch line from 20ft as drawn down to 8ft affect the airflow as described for that room.

I am under the assumption that the length(s) of the ducting is part of, not just the reason, of the Man D to help get the airflow down to the rooms requirement.

As dash mentioned, a shorter run will get more air. So this means a longer run will slow it down? If so, will having to take a branch line from 20ft as drawn down to 8ft affect the airflow as described for that room.

The velocity of the air is determined by the cross-sectional area of the duct.

Longer runs do impose more static pressure, but the difference between 20 feet and 8 feet is almost negligible. For the third or forth time, I lose count, shorten the runs and maintain the other dimensions the same as designed.